This invention relates generally to optical systems, and has particular reference to a novel optical system for illuminated viewing instruments. More specifically, the optical system of the invention is adapted for use in electrically illuminated diagnostic instruments such as ophthalmoscopes and the like.
In illuminated diagnostic instruments used in the examination of restricted body passages or other small openings, it has always been difficult to construct the instrument so that the viewer's line of sight substantially coincides with the beam of light emitted by the instrument. This has been particularly true of eye instruments such as ophthalmoscopes where the pupil provides the only natural opening through which the interior of the eye can be observed.
Even when dilated, the pupil is still a relatively small opening and there are many times when the fundus of the eye must be examined but dilation is not possible. Thus, the eye can only be dilated with caution when the patient is on medication for glaucoma, or when the patient is a baby or small child, and in certain other situations such as an emergency when time may not permit dilation. As will be readily understood, if there is anything more than a small angular displacement between the light beam and viewer's sight line, the viewer will not be able to look at the center of the area being illuminated.
Heretofore, the optical systems of ophthalmoscopes utilized a prism for changing the direction of the illuminating beam from axial with respect to the instrument to normal so that it can be projected outwardly into the patient's eye. This is necessary in this type of instrument since it is not practical to try to mount the light source in the upper part of the instrument in line with the patient's eye. Since the viewer cannot see through the prism, his line of sight must be just over its top and this means that the light from the light source must be made to emerge from the prism as close to the top as possible so that it almost coincides with the line of sight. This requires a precision prism, and even in the most precise instruments the optimum condition of having the line of sight and light beam coincide can never be attained.
At the present time, most ophthalmoscopes utilize a first surface mirror rather than a prism for changing the direction of the illuminating beam. In the majority of instrument designs this has reduced the angular displacement between the light beam and viewer's sight line but a further reduction is still needed, particularly when the pupil cannot be dilated. With the mirrors in present use, the viewer's line of sight still must be over the top.